I think this is a great idea for a fic, and I loved the scene in the middle between Tonks and Moody - it really showed a beginning for the relationship between them that we see in canon.

I think you have a canon error - at the beginning, you said that "If Tonks had just been one year younger, he would have been her teacher". This isn't technically correct - Tonks was in the same year as Charlie, who graduated before Harry started, so she would have had to have been 3 years younger to have been taught by Remus. I also found it a little strange that you never used Remus' first name. I know often in canon he is referred to by his last name, but that is because the books are basically Harry's perspective and for him Remus is a former teacher.

Those are just small things though, I really did enjoy this. "They had all known this might happen, and they were all willing to die in order to defeat Voldemort. But when Tonks thought about Moody, and the kind of man he was – he shouldn’t have died like that. He should have died where everyone could have seen and applauded him – not where his death would have to be covered up. Not where the Order had to go and rescue his body to stop the Death Eaters taking it." - I really loved that paragraph and how it showed both Moody's and Tonks' characters.

The only other thing is that I wish it had been a little longer. Perhaps you could have showed us more interaction between Tonks and Moody or something?